Simplify 'obj' property definition in PyobjMixin

This uses modern property definition syntax, declaring both getter
and setter as obj() functions

src/_pytest/python.py

@@ -243,8 +243,9 @@ class PyobjMixin(PyobjContext):
     def __init__(self, *k, **kw):
         super(PyobjMixin, self).__init__(*k, **kw)
 
-    def obj():
-        def fget(self):
+    @property
+    def obj(self):
+        """underlying python object"""
         obj = getattr(self, "_obj", None)
         if obj is None:
             self._obj = obj = self._getobj()
@@ -254,14 +255,12 @@ class PyobjMixin(PyobjContext):
                 self.own_markers.extend(get_unpacked_marks(self.obj))
         return obj
 
-        def fset(self, value):
+    @obj.setter
+    def obj(self, value):
         self._obj = value
 
-        return property(fget, fset, None, "underlying python object")
-
-    obj = obj()
-
     def _getobj(self):
+        """Gets the underlying python object. May be overwritten by subclasses."""
         return getattr(self.parent.obj, self.name)
 
     def getmodpath(self, stopatmodule=True, includemodule=False):